Electron discharge glow control electrode



Jan. 27, 1953 G. H. HOUGH ETAL 2,627,045

ELECTRON DISCHARGE GLOW CONTROL ELECTRODE Filed April 29, 1949 INVENTORSGEO/PGf H. HOUGH- BLYESL/E C. SKI/(t7? ATTORNEY Patented Jan. 27, 1953UNITED STATES PATENT OFFICE ELECTRON DISCHARGE GLOW CONTROL ELECTRODEApplication April 29, 1949, Serial N 0. 90,316 In Great Britain May 3,1948 6 Claims. 1

This present invention relates to cold cathode electric dischargedevices.

By cold cathode discharge devices we mean a device having an envelopefilled with an ionisable gas at reduced pressure and containing one ormore discharge gaps designed as to their gap lengths and dimensions tofunction as glow discharge devices as opposed to arc or spark dischargegaps.

As is well known, in cold cathode discharge tubes so long as the wholeof the available cathode surface is not covered with glow, the potentialacross the tube remains considerably below that required to initiate adischarge; the discharge in this region is referred to as normal. Whenthe whole of the available cathode surface is covered with glow,abnormal discharge commences with a corresponding rise of potentialwhich increases more than proportionately to the increase in cathodecurrent, until finally arcing occurs. For many applications it isrequired to limit the normal discharge from any iven cathode to aprescribed area; this is particularly the case in what are known assequence discharge tubes such as disclosed in the co-pending applicationof Alec H. Reeves, bearing Serial No. 15,582 and filed March 18, 1943.In such a device, a plurality of cathodes is provided with one or morecorresponding anodes and it is arranged that the ionisation associationwith the cathode glow at any one gap primes the neighbouring dischargegap so that the striking potential of that gap is reduced. When voltagepulses are applied across the gaps in common, discharges may thus be setup in sequence from cathode to cathode. For satisfactory operation ofsuch a tube as a counter for example, we prefer to work in the region ofabnormal glow but well below the arcing region; for this reason thecathodes are made small in area and heretofore nickel rods have beencoated with alumina or some similar discharge inhibiting substanceexcept for a small area comprising the required cathode surfaceimmediately facing the anode. A disadvantage of such a coating is thatat the edges of the discharge area, insulating material tends to beremoved during operation and furthermore sputtering of the conductingmaterial onto the surface of the insulator results in a gradual increasein the effective cathode discharge area which results in unstableoperation. In this connection it should be pointed out that for sequencedischarge tubes we have found it preferable to use the same materials,usually pure metal, for both cathode and anode and have found itnecessary to take great care of the purity of the gas filling andcleanliness of any other materials introduced in the tube. It shouldalso be pointed out that in such tubes although anode and cathode may beof the same material, completely different operating conditions resultif these electrodes are interchanged in circuit, for if only the anodedischarge surface is limited in area the cathode glow may wander allover the surface of a cathode during normal glow resulting inindeterminate ionisation coupling between gaps.

We have found that if a cathode electrode is surrounded by but separatedfrom material which may be either insulating such as mica ormetallic-provided the separation between the electrode and the materialis less than the extent of the cathode fallapproximately the distancebetween the cathode and the cathode glow in normal operationthe cathodeglow will not pass between the electrode and the material to anyappreciable extent until abnormal glow conditions have set in.Furthermore, even after this abnormal glow has occurred the restrictionof the glow continues for some distance on the characteristic curve ofthe gap.

It is pointed out that the present invention is not to be confused withthe procedure adapted in rectifier tubes for providing insulationagainst breakdown between conductors during the application of high peakinverse voltages. It has long been known that if two conductors in anionisable gas be separated by a distance less than the mean free path ofan electron in the gas, breakdown of the gap between them is prevented.For a typicalgas mixture which we use in embodiments of the presentinvention the mean free path is but 0.008 mm. and it is consideredimpracticable to use such small clearances for present purposes. On theother hand as the length of the cathode fall in the same gas is 0.18 mm.it becomes practicable to use clearances of this order.

According to the present invention therefore, there is provided a coldcathode gas filled electric discharge device comprising a discharge gapbetween a pair of electrodes adapted to be used as a cathode and ananode respectively and a non-contacting collar of insulating or metallicmaterial surrounding the cathode electrode at such distance from thesurface thereof (being a distance greater than the mean free path of anelectron in the gas) that cathode glow is restricted substantially toone side of said collar at least for discharge currents less than agiven value in the region of abnormal discharge conditions for said gap.

A collar is often most conveniently provided by a flat sheet of materialand the invention provides a cold cathode gas filled electric dischargedevice comprising a pair of electrodes forming a discharge gap and anon-contacting sheet of insulating or metallic material surrounding oneof said electrodes to form a collar spaced from the surface thereof by adistance greater than that of the mean free path of an electron in thegas but less than that of the cathode fall for normal cathode glowdischarge from that electrode.

According to a further aspect of the invention there is provided a coldcathode gas filled electric discharge device comprising a plurality ofdischarge gaps formed between separate cathode surfaces and at least oneanode electrode and a non-contacting sheet of insulating or metallicmaterial juxtaposed said cathode surfaces at said distance therefrom aswill permit abnormal glow discharge to occur before cathode glow extendsbeyond a limited area of each said surface.

According to yet another aspect of the invention there is provided a,cold cathode gas filled electric discharge device comprising an anode oranodes, an apertured sheet of insulating or metallic material and aplurality of cathode rods projecting through said apertures to formdischarge gaps with said anode or anodes, the clearance between saidrods and the surrounding said sheet being greater than the mean freepath of an electron in the gas but less than the extent of the normalcathode fall for each said discharge gaps.

The invention will be described with reference to the accompanyingdrawings, in which:

Fig. 1 shows a cross section through a typical discharge gap, and

Fig. 2 shows an electric discharge device of the sequence discharge typeaccording to the present invention.

In many of the standard text books (see for example L. B. Loeb TheFundamental Processes of Electrical Discharge Through Gases) the lengthof the cathode dark space for various gases is given, or it can bederived from the curve relating to the product of the gas pressure andthe gap length to the minimum striking voltage for a particular gas. Onsuch a curve the minimum value of the product of pressure and gap lengthcorresponds to the length of the cathode dark space.

Referring now to Fig, 1, reference numeral I indicates a part of theanode, which may be in the form of a wire or plate and 2 represents acathode which is assumed in this example to be cylindrical. 3 indicatesa sheet of metal or insulating material surrounding the cathode toprevent the spread of glow according to the present invention, while thedotted line 4 indicates the extent of the cathode dark space. Thus, fora gap length between anode and cathode of 1 mm., using a gas mixturecontaining 92% Ne, 1% A and 7% H2 at a pressure of 100 mm. of mercury,the cathode dark space extends approximately 0.183 mm. from thecylindrical cathode surface. We have found that provided the separationbetween the cathode surface and plate is less than this, say 0.125 mm.the cathode glow is inhibited from spreading below the plate even thoughthe discharge current is taken well into the abnormal glow region. Forhigher values of abnormal glow the cathode dark space becomes smallerand eventually the glow will spread through the aperture but this isrelatively unimportant because even then a stable glow is still obtainedabove the plate. Thus for a cathode rod of 1 mm. diameter with ananode-cathode gap of 1 mm. we have found that with the aperture spacingas given above a current of '7 Ma. can be drawn from the cathode beforethe glow spreads below the plate to an appreciable extent. It will beseen that the plate is effectively a field control member and owing tothis fact, and provided the potential of the plate if of metal be lessthan required to maintain a discharge to the cathode, its effect uponthe operation of the discharge between anode I and cathode 2 will benegligible except for its purpose of confining the glow to the regionabove the plate.

A practical construction of a sequence discharge device suitable for useas a counter is shown in Fig. 2. Cathode rods 5 are mounted by means ofthe lead-in wires 6 on a conventional glass press I forming part of theenvelope 8 of the device. A single anode 9 and a field control plate IOare mounted by means of mica separators H secured in normal manner viasupport rods 12 to the press 7. It will be seen that only the tip of thecathode rods 5 protrude through the field control plate In.

By means of the present invention it has been possible to eliminate fromthe inside of the discharge tube all materials other than the puremetals of the electrodes, glass, mica and the necessary gas filling. Inconsequence the effect of ageing is reduced, operation is stable andcathode sputtering leads to little loss of tube life. A tube such ashere described has been used to count 1 a sec pulses of 1.3 Ma. currentat a repetition rate of 10 kc./s. continuing for more than 2000 hourswithout any measurable change in characteristic.

While the principles of the invention have been described above inconnection with specific examples and particular modifications thereof,it is to be clearly understood that this description is made only by Wayof example and not as a limitation on the scope of the invention.

What is claimed is:

1. A cold cathode electric discharge device com prising a gas-filledenvelope, an anode, cathode electrode means having a first portiondefining a discharge gap with said anode and having a second portionadjacent said first portion, discharge field control means surroundingsaid cathode means and spaced in a first direction from the firstportion of said cathode means a distance greater than the mean free pathof an electron in the gas; said field control means spaced in a seconddirection from said second portion of said cathode means a distance lessthan the oathode dark space, whereby cathode gloW is restrictedsubstantially to the area between said field control means and the firstportion of said cathode means when discharge currents of less than agiven value in the region of abnormal discharge conditions are passedthrough said gap.

2. A cold cathode electric discharge device comprising a gas-filledenvelope, an anode, a plurality of spaced cathode electrodes, eachdefining a separate discharge gap with said anode, discharge fieldcontrol means surrounding each of said electrodes and spaced from thoseportions of said electrodes which define said gaps, said spacing beinggreater than the mean free path of an electron in the gas, wherebycathode glow is restricted substantially to an area between said fieldcontrol means and the portions of said electrodes defining said gapswhen discharge currents of less than a given value in the region ofabnormal discharge conditions for said aps are passed therethrough.

3. A cold cathode electric discharge device as claimed in claim 2,wherein said discharge field control means comprises a multi-aperturedplate, each of said cathode electrodes projecting through a differentaperture in said plate, the clearance between each cathode electrode andthe sides of the material surrounding its associated aperture beinggreater than the mean free path of an electron in the gas but less thanthe extent of the normal cathode fall for each said discharge gap.

4. A cold cathode electric discharge device as claimed in claim 2,further comprising insulating supporting means, wherein said cathodeelectrodes are disposed in parallel alignment and are maintained inposition by said supporting means.

5. A cold cathode electric discharge device as claimed in claim 4,wherein said supporting means comprises a plurality of sheets ofinsulating material supported within said envelope, said cathodeelectrodes being rigidly held by said sheets.

6. A cold cathode electric discharge device comprising a gas-filledenvelope, a plurality of mounting rods supported by said envelopetherein, a plurality of insulating sheets mounted on said rods, each ofsaid sheets mounted parallel in plane with the other and spaced fromeach other, an anode supported by at least one of said sheets, aplurality of cathode rods disposed 6 in parallel alignment, each of saidcathode rods defining a separate discharge gap with said anode, amulti-apertured metallic field control plate supported on said mountingrods in a plane parallel to the plane of said insulating sheets, saidplate disposed between a pair of said sheets and spaced therefrom, eachof said cathode rods projecting through a dilferent aperture in saidplate, the clearance between each cathode electrode and the sides of thematerial surrounding its associated aperture being greater than the meanfree path of an electron in the gas but less than the extent of thenormal cathode fall for each of said discharge gaps, whereby a cathodeglow is restricted substantially to an area between said plate and theend of said cathode rods defining said discharge gaps when dischargecurrents of less than a given value in the region of abnormal dischargeconditions for said gaps are passed therethrough.

GEORGE HUBERT HOUGH. LESLIE CHARLES BAKER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,877,716 Claude Sept. 13, 19322,098,301 Mendenhall Nov. 9, 1937 2,331,398 Ingram Oct. 12, 19432,373,175 Depp Apr. 10, 1945 2,443,407 Wales June 15, 1948 2,456,854Arnott et al. Dec. 21, 1948 2,487,437 Goldstein et a1 Nov. 8, 1949

